1. Field of the Invention
The present invention is in the field of telecommunications and, in particular, in the field of signal transmission over a communication network, e.g. internet.
2. Description of the related art
Video streaming over the internet—or any other kind of network where the resources are shared by many users—always faces the problem that at one or more points along the path the incoming rate at a network node might be higher than the outgoing rate. This leads to an increasing buffer-fullness of that node and eventually to packet loss. For video signals, if more traffic arrives than the outgoing links can support, the video data either has to be transcoded to lower rate or video packets have to be dropped.
Transcoding is computationally expensive and random frame dropping can have a negative influence on the video quality. Scalable video offers the opportunity to drop less important parts of the video bit stream first, which leads to graceful degradation as traffic increases.
Quality of service (QOS) labelling of the video packets together with priority mechanisms in the network node support importance-controlled dropping of the data. The label (or importance) of the packets is determined by the sender before transmission and does not include the actual transmission situation. This is a disadvantage as the importance of a packet might change along the transmission path.
In the following and by the way of example, a video stream with temporal scalability will be considered which has the following group of picture structure: IBBPBBP . . . If a network node drops a B-frame the other frames are not affected. If, however, the P-frame after the I-frame is dropped, all following frames (B- and P-frames) up to the next I-frame will be affected as they depend on the dropped frame. Therefore, if it is known that the first P-frame has to be dropped, then the importance of all following frames changes. Although, usually, only a few different importance labels are present, different frames with the same label will still have different influence on the reconstruction quality at the receiver.
As it is known from the rate-distortion theory by C. Shannon, an information distortion increases as the rate decreases. In this context, the term “distortion” refers to some measure of a difference between e.g. samples of a signal having a certain rate and samples of the signal after rate reduction.
If, for example, a certain frame within the group of pictures is dropped, then an increased distortion associated with rate reduction achieved by frame dropping can be observed. At a receiver, an information loss associated with the missing frame can at least partially be compensated at the receiver when a concealment strategy, e.g. reconstructing the missing frame by introducing e.g. a previously received frame comprising some information also comprised by the missing frame. Therefore, after applying the concealment strategy at the receiver, a resulting reconstruction distortion may be different than the distortion when not applying any concealment strategy. In the following, the term “reconstruction distortion” denotes a resulting information distortion observed at the receiver after applying a concealment strategy, e.g. replacing missing frames by other frames.
Ralph Keller, Sumi Choi, Dan Decasper, Marcel Dasen, George Fankhauser and Bernhard Plattner, describe in “An Active Router Architecture for Multicast Video Distribution,” Proc. Infocom 2000, Tel Aviv, Israel, March 2000, a dropping strategy for scalable video that can be implemented on active routers. However, the dropping decision is not made in a rate-distortion optimum way, since after applying the dropping strategy disclosed therein, the distortion may significantly be increased. G. Ravindra, N. Balakrishnan, K. R. Ramakrishnan disclose in “Active Router Approach for Selective Packet Discard of Streamed MPEG Video under Low Bandwidth Conditions.” Proc. ICME 2000, New York, July 2000, a video frame dropping strategy that discards all those frames that depend on previously dropped frames. However, the dropping decision does not consider many simultaneous video streams in a rate-distortion optimum way, so that certain video streams suffer from significantly higher distortion than other video streams.